In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view" A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object.
One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time required for multiple slices, a "helical" scan may be performed. To perform a "helical" scan, the patient is moved in the z-axis synchronously with the rotation of the gantry, while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed. In addition to reduced scanning time, helical scanning provides other advantages such as better control of contrast, improved image reconstruction at arbitrary locations, and better three-dimensional images.
Efforts have been undertaken to enhance the quality of inner auditory canal (IAC) structure images. The most significant image quality issues with IAC structures include a lack of "sharpness" in the IAC structure and excessive aliasing artifacts that obstruct human anatomy. With a third generation scanner, if the detector and x-ray focal spot response are modeled as square waveforms, it can be shown that two samples within each detector cell width are required to eliminate aliasing artifacts.
In some known third generation scanners, it is not possible to obtain such samples. Other known scanners employ x-ray focal spot wobbling in an attempt to obtain sufficient samples. Tube design complexity, tube reliability, detector temporal response, and resources impact concerns all arise when focal spot wobbling is employed.
Still other known scanners employ quarter detector offset in an attempt to reduce aliasing artifacts. Particularly, by aligning the iso-center of the system and the center of the detector a quarter of a detector cell apart, interleaved samples can be obtained near the detector center when 2.pi. views of projection data are acquired. However, quarter detector offset is limited in that the data interleaving is only near perfect at the detector center, and at locations spaced from the detector center, the sampling pattern is not perfectly interleaved. Therefore, quarter detector offset generally only is effective at eliminating aliasing artifacts near the iso-center.
In addition to eliminating, or reducing, aliasing artifacts, it generally is desirable to enhance the "sharpness" of an image. Image enhancement techniques, such as highlighting the edges of an image, are known. However, such techniques, while enhancing "sharpness", also tend to increase image noise and aliasing artifacts. As a result, known image sharpness enhancement techniques sometimes reduce the overall image quality.
Particularly with IAC images, a high level image sharpness and a low level of aliasing artifacts are desired. It also is desirable to increase image sharpness and decrease the level of aliasing artifacts without reducing overall image quality.